The present invention relates to a variable-speed generator/motor system, or more in particular to a variable-speed generator/motor system which is capable of continuing to stably operate even under a disturbance caused by a fault or the like of an AC system.
A variable-speed generator/motor system has a well-known configuration as disclosed, for example, in U.S. Pat. No. 4,481,455. The primary winding of the generator/motor is connected to a power system through a main circuit including a circuit breaker and a main transformer, and the secondary winding which is installed on the generator/motor rotor directly coupled to a prime mover/load, is connected to the power system through a frequency converter and an excitation transformer. This system, which has a great advantage in that the rotational speed of the prime mover/load may be set regardless of the frequency of the AC system, already finds some applications for variable-speed pumped-storage power stations and flywheels. In applications to the variable-speed pumped-storage power station, for instance, the secondary winding of the generator/motor is installed on the rotor directly coupled with a water turbine/pump as a prime mover/load and excited by the frequency converter, so that the primary winding of the generator/motor (generator) is energized by the water turbine in power generation mode thereby to supply the power system with the power generated in the primary winding thereof. In motor (pumping) mode, on the other hand, the generator/motor is driven as a motor by the power from the power system, and water is pumped by a pump directly coupled to the rotor. In this case, in spite of the power system frequency which is fixed to, say, 60 Hz, the rotational speed of the water turbine/pump can be selected as desired independently, and it is thus possible to set the water turbine-pump to a rotational speed which maximizes the operating efficiency thereof.
The fact that a desired rotational speed can be set for the water turbine/pump is indicative of the fact that there is a frequency difference f.sub.2 between the frequency f.sub.1 of the power system and the frequency f ##EQU1## P: Number of poles) of the prime mover/load that is associated with the number N of r.p.m. Equation (1) is thus obtained. ##EQU2## The frequency f.sub.2 of equation (1) is that of the secondary winding current, and it is possible to maximize the efficiency of the pump or water turbine while keeping the frequency f.sub.1 of the power system at a fixed level. There are two main operating factors adjustable to satisfy the relationship of equation (1). One is the opening of a quide vane on the water path of the water turbine/pump, and the other is the firing angle of a thyristor making up a frequency converter. These operating factors are appropriately controlled by a target signal determined by the object of operation, etc. of the variable-speed pumped-storage power station involved, (for example, by using as a target signal or a load demand signal from a central load dispatching office), even in which case the relationship of equation (1) is held.
The firing angle control of a thyristor requires a sync signal in phase with the frequency of the AC system connected to the thyristor in addition to a control signal determined from the target signal. A method of detecting a phase signal as such a sync signal is well known as disclosed in Japanese Patent Publication No. 28613/75, in which the secondary winding of a wound-rotor induction machine for phase detection (phase detector) is arranged in axial alignment with the secondary winding of the main apparatus (generator/motor), the primary winding of the wound-rotor induction machine is connected to the power system in the same manner as the primary winding of the main apparatus, and the signal from the secondary winding of the wound-rotor induction machine is recovered as a sync signal. In this method, the wound rotor induction machine for phase detection and the main apparatus are connected to the same power supply, and therefore in the case of a system disturbance such as a frequency drop, the phase detector making up the wound-rotor induction machine detects a slip, thereby decreasing the output frequency of the main apparatus for aggravation of the disturbance.
To obviate the problem caused by a frequency drop, Japanese Patent No. 1161562 entitled "Pulse Phase-Shift Control System" suggests, taking an AC-DC converter for DC power transmission as an example, "a pulse phase control system comprising a voltage-regulation oscillator supplied with an AC system voltage as an input applied to an AC-DC converter and producing a sync signal in phase with the input, the system producing a firing pulse of a phase corresponding to the control signal, in which a signal associated with the phase of the system voltage immediately before a fault, if any, of the AC system is stored, and during the continuance of the fault of the AC system, the signal thus stored is used to energize the voltage-regulation oscillator". A function similar to this invention is required to be added also to a variable-speed generator/motor system. It is, however, impossible to do so without modification of the above-mentioned prior art system. Specifically, unlike in the cited patent wherein the voltage phase applied to the voltage regulation oscillator is that of the AC system voltage applied to the AC-DC converter, a signal generated in the secondary winding of a generator/motor (the signal of frequency f.sub.2 in equation (1)) is applied to the frequency converter in the case of the variable-speed generator/motor system. According to the phase detector of Japanese Patent Publication No. 28613/75, the secondary voltage of a wound-rotor induction machine is stored as a sync signal. In determining a firing angle of the frequency converter by storing the voltage of frequency f.sub.2, it is impossible for the frequency of the main circuit of the, generator/motor to coincide with that of the power system after a disturbance as will be explained later with reference to FIG. 6.
In spite of the foregoing explanation of a variable-speed generator/motor system, the generator/motor functions as a motor when the prime mover/load is used only as a load. This configuration is known as what is called the Scherbius system, to which the present invention is also applicable. In the description that follows, these systems will be collectively referred to as a variable-speed generator/motor system.